Known perimeter security sensors or intrusion detection systems utilizing open transmission lines incorporate a source of radio frequency energy as a component of the system. This can be used to set up a field around a transmission line which is monitored by a second parallel line or to set up a field from a central antenna which is monitored by an open transmission line. Guided radar type of intrusion detection systems have been developed using leaky coaxial cables. In most guided radar systems there are two parallel cables. One is used to distribute an electromagnetic field along the desired pathway and the parallel receive cable is used to monitor the field coupling between the two cables and thereby to detect movement of people or objects which disturb the coupling. Both continuous wave (cw) and pulsed type guided radars using leaky coaxial cables have been developed. Canadian patents numbers 1,216,340 and 1,014,245 by Keith Harman et al, both of which are incorporated herein by reference, describe two such guided radar systems.
An alternative form of guided radar which uses a leaky coaxial cable to monitor the field set up by a central antenna is disclosed in Canadian patent No. 1,169,939, by Keith Harman et al. In order to minimize the number of nuisance or false alarms, this system tracks the phase angle which changes as the intruder crosses the cable.
A disadvantage of both such systems is that the transmitter which generates the field is part of the system and in general requires radio regulatory approval.
The present invention seeks to overcome this disadvantage and to this end contemplates the use of an independent transmitter, for example an existing commercial radio or television station, as the source of the field which is subsequently used to detect intruders or other moving objects.